The present invention relates to a pulley device for a continuously variable transmission.
Continuously variable transmissions are known which use, as a continuously variable ratio change unit, a driver pulley, a driven or follower pulley and a V-belt interconnecting these pulleys. There is a growing demand for reduction in overall size of the continuously variable transmissions of this kind. Reduction in size of the pulley device, particularly its servo motor, is demanded.
U.S. Pat. No. 4,494,942, issued to Hirano et al. on Jan. 22, 1985 discloses a continuously variable transmission including a continuously variable ratio change unit. According to this continuously variable ratio change unit, a driver pulley and a follower pulley are interconnected by a V-belt. The driver pulley comprises a stationary conical pulley component including a shaft portion, a moveable conical pulley component mounted to the shaft portion for rotation therewith and for movement relative to the shaft portion in such a direction as to vary a distance between the conical pulley components. The stationary and moveable conical pulley components have conical faces opposed to each other for fricional engagement with the V-belt. In order to vary the distance between the opposed conical faces, a stationary piston is fixed to the shaft portion, and the moveable conical pulley component has an integral cylinder slidably engaging the stationary piston at an outer periphery thereof to define a fluid chamber on the opposite side of the moveable conical pulley component to the conical face thereof.
JP-A 3-189446 teaches to provide a boost piston. According to this teaching, a stationary piston includes a tubular portion radially opposed to a cylinder fixed to a moveable conical pulley component, an auxiliary stationary piston is fixed to the shaft portion and extends radially outwardly to slidably engage the cylinder, and a boost piston is disposed between the two stationary pistons. The boost piston is annular and has an outer periphery fixed to the cylinder and an inner periphery slidably engaging the tubular portion. The boost piston, auxiliary stationary piston, tubular portion and cylinder cooperate with each other to define a boost fluid chamber. According to this teaching, the cylinder with increased axial dimension is needed. Besides, the stationary pistons are in slidable engagement with the common cylinder. Thus, a considerable space is needed for accommodating this cylinder.